type PromptFunction

Description

A PromptFunction is used as a callback by functions that may need to decrypt
a private key, or prompt for a passphrase. It is called with a list of
acceptable, encrypted private keys and a boolean that indicates whether a
passphrase is usable. It should either decrypt a private key or return a
passphrase to try. If the decrypted private key or given passphrase isn't
correct, the function will be called again, forever. Any error returned will
be passed up.